1. Field of the Invention
The present invention relates to a method for forming an oxide film in a semiconductor device, and more particularly, to a method for forming an oxide film in a semiconductor device which is suitable to form oxide films of different thicknesses in a device region, to which driving voltages of different levels are applied respectively.
2. Discussion of the Related Art
In general, an embedded DRAM, or a semiconductor device with different driving voltages requires oxide films having thicknesses different from each other. Of the DRAMs and logic devices respectively developing rapidly as integration of semiconductor devices advances, with emerging of the embedded DRAM which is a combination of the DRAM and logic device, which trend is expected to be continuous in the future, regions with different driving voltages requires oxide films with thicknesses different from each other, such as memory cell regions, core and peripheral regions in the DRAM, regions of the logic device excluding input/output regions of the logic device, input/output buffer regions of the logic device and ESD (Electro Static Discharge) regions of the DRAM and the logic device. The oxide films with different thicknesses in a semiconductor device may be formed by wet etching or by ion injection. However, because the above two methods have problems in view of uniformity of the thickness and reliability of the oxide films, various researches for solving such problems are underway, actively.
A background art method for forming an oxide film in a semiconductor device will be explained with reference to the attached drawings. FIGS. 1a.about.1e illustrate sections showing process steps of a background art method for forming an oxide film in a semiconductor device.
Referring to FIG. 1a, a sacrificial oxide film 2 is formed on a semiconductor substrate 1 in regions defined as a first region A a thick oxide film to be formed thereon and as a second region B a thin oxide film to be formed thereon, respectively. As shown in FIG. 1b, a photoresist film PR is coated on the sacrificial oxide film 2 and subjected to patterning by exposure and development to leave the photoresist film PR only on the semiconductor substrate in the first region A. As shown in FIG. 1c, nitrogen ions 3 are injected into the semiconductor substrate 1 in the second region B using the photoresist film PR as a mask and subjected to annealing, to diffuse the nitrogen ions 3. As shown in FIG. 1d, the photoresist film PR and the sacrificial film 2 are removed. As shown in FIG. 1e, the entire surface of the semiconductor substrate 1 is subjected to annealing, to grow a thick first gate oxide film 4 on the semiconductor substrate 1 in the first region A and a thin second gate oxide film 5 on the semiconductor substrate 1 in the second region B. That is, because of the slow growth of the gate oxide film in the portion of the semiconductor substrate 1 into which the nitrogen ions are injected than the portion of the semiconductor substrate 1 into which no nitrogen ions are injected due to suppression of oxidation at a surface of the semiconductor substrate by the nitrogen ions, a dual gate oxide film is formed, in which the first, and second oxide films 4 and 5 have different thicknesses. And, though not shown in the drawings, a fluorine ion injection instead of the nitrogen ion injection will cause a result opposite to the aforementioned result. That is, the fluorine ion activates oxidation at the surface of the semiconductor substrate, to form an oxide film thicker than other region to which the fluorine ion is not injected. In the background art, either nitrogen or fluorine ions are injected into a semiconductor substrate in formation of the dual gate oxide film.
However, the background art method for forming an oxide film has the following problems.
First, the thin gate oxide film by nitrogen ion injection has a low reliability as a gate oxide film due to a poor TDDB (Time Dependent Dielectric Breakdown) characteristic.
Second, in the case of a gate electrode with a dual polygate on a thick gate oxide film by fluorine injection, the boron ions in p.sup.+ type polysilicon (gate electrode) diffuse into the semiconductor substrate (particularly into a channel region), which causes a problem of a low reliability of a transistor.